Lee Hambley

Founder, Consultant, Software Engineer

Hamburg, Germany

You might think that GNU's Make is a tool for unix beards to compile the C programs that you can't read, and never seem to work for you, but there's a chance you've heard it's also pretty good for so-called workflow control.

One of the key things about Unix was supposed to be that everything is a file, and as a result a lot of things input, and output files as their native language, we can use this to our advantage, given the following problem:

  1. We need a SQL file from the web, it's gzipped (and for some reason it's z7 zipped, but with .tar.gz file extension!).
  2. We should grab it if we don't have it already, and verify it's signature.
  3. We should unzip it.
  4. We should then import it to the SQL server.
  5. We can then run our job on it.

We could solve this with some shell-scripting, or some if-else-etc constructs, or - we could better rely on make to guarantee all these things happen.

Make will recognise that a prerequisite exists already, and will skip that step of the workflow, if it does not need to be performed again

That means, when you've waited once for the download to complete (see code below), once that file exists, you can re-run make, which will recognise that the file already exists and skip that step.

Here's a sample Makefile from one of my projects:

import: | import-sample
                download: data/latest-seed.sql
                
                import-sample:
                  @./import.sh 150
                
                import-all:
                  @./import.sh
                
                clean:
                  rm -rf tmp/*
                  rm -rf export/*
                  ./bin/clean_db.sh
                
                data/latest-seed.sql: tmp/June-16-2012.z7
                  @mkdir -p $(@D)
                  @7z e $< -so > $@
                
                tmp/latest-seed.z7: tmp/latest-seed.rar
                  @cp $< $@
                
                tmp/June-16-2012.rar:
                  @mkdir -p $(@D)
                  @curl "http://www.example.com/latest-seed.rar" -o $@
                
                .PHONY: import-all import import-sample clean download
                

A breakdown, lines with no indentation define targets, anything after the semi colon defines a target that must be built before we can built this target (which may in turn define it's own prerequisite targets).

The lines beginning with indentation are implementation lines, the @ swallows the shell output.

The mixture of @D, $@, $< and friends are various automatic variables, they refer to parts of the definition, so that we can reuse them, the documentation isn't too difficult to read, but the ones I've used here are

@D
The directory part of the task name. (if any)
$@
The name of the prerequisite.
$<
The name of the target's first prerequisite.

With the details out of the way, it's worth noting that you can also pass them to any shell commands, in Makefiles you have to use the syntax $(shell .......) to shell out, as $(..) is the variable syntax.

That said, one could shell out to sed, awk and friends with $(shell ...), wherein any variables as noted above will be available.

Note: I didn't need to export any environmental variables here, but if I did, the syntax is that they have to be written at the top of the file (usually) and they shoul have the syntax RACK_ENV := development, or something, for comparison that would be export RACK_ENV=development if you did it in a shell script. Again, the distinction between the usual way, and the Makefile way is because of Make interpreting the syntax we usually associate with shell scriping in it's own way.

The first target line looks a little strange, the <target>: | <prereq>, (empasis on the pipe) syntax means this is an order only depednency it doesn't have any file-output that should be checked, it's just a task, something that will always need to be done. That way, if all file prerequisites are called make will run run the shell script.

The last thing that's unusual about this makefile is a relatively marge number of .PHONY tasks. The .PHONY directive shows make that all the following tasks are always invalid and always need to be run again. This way make clean will always run regardless of any of make's own inferences of what ought to run.

In summary, this workflow has saved me a lot of time, and whilst tools such as Rake (Ruby make) might fit better into my workflow, I often use make to grab seed files, to unpack, verify, sign and check files which are very often fed into Ruby, or Go language programs. Make is an excellent way to do dependency driven shell scripting, and it absolutely has a place along side other tools in your stack. I used use makefiles to prepare the environment for my Go programs to run. (Infact, in the example above the import.sh makes some SQL schema changes that I couldn't script into Make, and then runs a go program to export the data as JSON which my Ruby app expects, talk about Rube Goldberg being a polyglot!

Make has a lot more to offer than you might expect, but it is also cursed with a LOT of magic, such as how an empty makefile in a directory of C (and other languages) will do the right thing, based on decades of magic that has been baked in, somtimes reading someone elses makefile can be challenging, when there's magic you didn't know existing running for reasons you don't understand in a language you aren't familar with, it can be challenging, but it's a handy tool for directing workflow non the less, and it's very approachable, just don't expect to get much out of other people's rakefiles.

Hat-tip to Ted Dzubia, who blogged about this back in Febuary 2011, and inspired me to embrace Make along side all my higher-level, and more modern tools.

Ted took down his blog following a personal change of heart in 2012, but he was kind enoguh to grant someone permission to mirror the site, and you'll find the post that inspired me at:

This work is licensed under a Creative Commons Attribution 3.0 Unported License.

Licences for my individual projects, and mini-code snippets can be found within each.